Custom Torsion Bars?

[Scooter]

Okay, just thinking outside the box....

Let's say someone has a G50 car that they want to backdate. Let's also say that they want the torsion bar hole to have the same visual appearance as a 915 car. As you know the G50 torsion bar sits out wider than a 915, thereby necessitating the larger hole...that is, if you ever want to remove/install torsion bars.

So, the question is this...what would be the issues with having a custom torsion bar made that would be short enough to use a 915 spring plate on a G50 car?

I know coil-overs are an alternative, but I am looking for a discussion on the described question at hand.

There you have it. Now, talk amongst yourselves.



Oh, and thanks in advance for your respectful input!

[jpnovak]

Scott,

I guess it would depend on the spring rate. I would be concerned that the degree of twist rotation would be an issue in a shorter bar putting a huge amount of stress. I know this can be calculated but I am not the one to do that.

There has got to be a way to fill that slot in the QP such that it looks a lot cleaner.

[jpnovak]

Sway away does make custom drive axles. Call them and ask about torsion bars.

[Scooter]

Jamie, thanks for the input. I have a friend that is an engineer and he thought the torsion bar may have a shorter life due to the stress level, but the theory is unsubstantiated at this time.

I have not looked at Sway-A-Way, but I do know there are companies that make custom torsion bars.

[gtc]

Bilstein RSR rear shocks

You're welcome.

[Flieger]

A shorter bar gives you a stiffer bar (holding diameter constant) but also means a lesser maximum load (meaning that if you make the rate equal by playing with the diameter then you have less rotation available with the shorter bar). The stronger your spring material the shorter your bar can be for a given suspension travel and load requirement.

You can think of it this way: strain energy (deflection) is like heat, and metals can only withstand a certain temperature before they melt. The more mass something has (like a brake rotor) the more heat it can absorb per degree of temperature rise. The less material you have (as with a hollow bar) the more highly stressed the material is.

[Evan Fullerton]

Short rear torsion bars have been made before. There are a few 911 race cars out there with VW bug style spring plates which would make them 3'' or so shorter then a standard 915 911 torsion bar. Seems to work pretty well for them so I would think that you should be fine using a 915 spring plate on a g50 car with a custom short torsion bar as long as you went with a suitably large diameter bar. I to have thought about doing it as well ad a friend and I have both put g50 torque tubes in our 915 chassis cars but are currently running coil-overs.

[chris_seven]

If you shorten a typical 25mm dia torsion bar by 3" you will only reduce the diameter by around 0.75mm to give an equivalent stiffness.

This would increase the shear stress by around 10% which is probably within the fatigue capability of a standard bar. (Torsional Stiffness is proportional to the 4th power of diameter and torsional stress is proportional to the 3rd power of diameter)

If you are making custom bars and you specify a material such as 300M you will gain much more than a 10% increase in the fatigue endurance limit compared to the material used to make a standard bar and all should be good.

If you buy a cheap bar out of poorly specified material you will already be reducing safety factors so an increase in stress could be an issue.

Buy a good quality bar and IMHO you will be OK.

[Trackrash]

VW Beetles had three different lengths of torsion bars over the years. I'm not aware that there were issues with the shorter bars.

[Scooter]

Now things are getting interesting.

Great input so far!